The obesity epidemic has spared no age group, including our youngest children. Overweight in preschool age children is associated with cardiovascular risk factors and asthma, and predicts overweight later in childhood. In addition, behaviors related to dietary intake, physical activity, and inactivity are likely "encoded" at these early ages. One relatively understudied site for behavior change is the primary care clinician's office. The overall goal of this research is to assess an innovative, sustainable primary care practice change intervention to prevent obesity among young children. To achieve this goal, we will conduct a cluster-randomized controlled trial in 10 pediatric practices of Harvard Vanguard Medical Associates, a large multi-site group practice in eastern Massachusetts with a track record of research collaboration. The trial will include 400 children age 2-5 years at elevated risk of obesity based on their and their parents'body mass indexes. The primary aim is to assess the extent to which the intervention, compared with the usual care control condition, reduces change in body mass index over a 6-month intervention and 2-year follow-up period. We will also assess the intervention's effect on specified obesity-related behaviors, its feasibility and acceptance, and its cost-effectiveness. Major features of the intervention include a primary care team approach based on the Chronic Care Model: a clinician-delivered intervention to modify behaviors of parents and children that employs motivational counseling, tailored to readiness to change: and outreach to child care settings as a reinforcing strategy. Behavioral goals for the children include reduction in TV/video viewing: fewer TVs in children's bedrooms: and reduction in sugar-sweetened beverage and fast food consumption. A need exists to combine the potential effectiveness of obesity prevention in the clinical setting with feasibility and sustainability. Through innovative methods and by extending existing collaborations among established childhood obesity researchers, experts in behavior change and cost-effectiveness analysis, and clinician-leaders of a large multi-site medical group practice, this project is poised to provide a new and sustainable paradigm for childhood obesity prevention in primary care.